Steam generators of fast neutron nuclear reactors often employ a liquid metal, such as sodium, as the heat-carrying fluid for the heating and the vaporization of the feed water by means of the heat taken from the core of the reactor.
These steam generators are often constituted by an outer casing having a generally cylindrical shape disposed with its axis vertical and connected at each of its ends to a thick tube plate. The exchange part of the steam generator is formed by a bundle of tubes placed in the outer casing inside a bundle case coaxial therewith. Each of the tubes of the bundle is connected by welding at one of its ends to the lower tube plate and at its other end to the upper tube plate in the extension of a corresponding bore respectively passing through the lower tube plate and the upper tube plate. The lower tube plate separates the inner volume of the casing from a water box permitting the distribution of the feed water in the tubes of the bundle. The upper tube plate separates the inner volume of the casing from a steam manifold receiving the steam formed in each of the tubes of the bundle.
The heat-carrying liquid metal enters the inner volume of the casing in its upper part, i.e., a little below the upper tube plate. The bundle casing has its upper end located at a certain distance below the upper tube plate so that the liquid metal enters the bundle casing above the upper end of this bundle case. In order to achieve a distribution which is as homogeneous as possible of the sodium along the tubes of the bundle, the upper part of the bundle casing constitutes a flow sill having a generally hollow rounded shape.
In order to avoid a direct contact between the lower side of the upper tube plate and the hot liquid metal entering the casing of the steam generator, there is often disposed, parallel to the tube plate and a little below the latter, a thermal protection plate which is thinner than the tube plate. The tubes of the bundle extend through the thermal protection plate in openings machined in the latter and are welded at their upper end to hollow spigots machined in the tube plate.
Between the thermal protection plate and the upper tube plate, the tubes of the bundle are surrounded over a part of their length by protection tubes having an inside diameter which is such that there is a radial clearance between the tube of the bundle and the corresponding protection tube. The radial space between the tube of the bundle and the protection tube opens, at one of its ends, onto the inner space of the steam generator below the thermal protection plate and, at its other end, onto the space between the thermal protection plate and the upper tube plate.
It is extremely important to detect very rapidly any possible leakage of feed water or steam in the heat-carrying liquid metal. Indeed, in the case of sodium, for example, a leakage of water or steam whose pressure is higher than that of the sodium may result in a very violent chemical reaction accompanied by a shock wave in the inner volume of the steam generator. It is possible to reduce or eliminate the disastrous effect of such a leakage if its detection is sufficiently rapid to permit the taking of the desired safety measures before an appreciable amount of water or steam has entered the liquid metal.
Such leakages have a much higher probability of occurring in the zones of the welding of the tubes to the tube plates, at their ends, than in the other zones of the tube. More precisely, the leakages often occur in the region of the weld between the upper end of the tube and the upper tube plate, this end of the tube being in contact with a liquid sodium which is much hotter than the lower end.
Devices for sampling liquid sodium have therefore been proposed for purposes of detecting and measuring the amount of hydrogen in the sampled sodium disposed in the vicinity of the upper end of the steam generator. However, these samples, which permit the detection of the presence of hydrogen revealing a leakage of water in the liquid sodium, are taken in a zone in which the hot sodium entering the steam generator arrives, so that they do not really concern a flow of sodium which has circulated in contact with the zone of the welding of the tubes and is consequently highly charged with hydrogen in the event of a leakage in one of the tubes.
The detection is therefore uncertain and depends on the rapidity with which the hydrogen spreads in the liquid sodium circulating in its upper part.